If you go to the free liveries section and you will find a weapons pack (the second entry I think, by Chris Sykes). If you download this I am sure it contains one. You will have to make the new entry in the .xml file and move the model folder etc. yourself, but just copy the standard layout of the existing weapons and you will be fine.

To make the effects bigger you will have to look to the effects files - I have no idea what to adjust but thats where the data is.

Pete